Detect Traps is a core rogue ability learned at level 42. It allows rogues to find traps laid out by others. When paired with [Disarm Trap], a rogue becomes essential for the safekeeping of any band of dungeon explorers.
Many rogues have expressed great displeasure over the change of Detect Traps to a passive in patch 1.11 (Shadow of the Necropolis). They claim that they enjoyed spamming the "Swirly Ball" (a reference to the animation of the old, castable spell), and now have "nothing to do during buff downtime at raids".
Patch 1.11.0 (20-Jun-2006): Detect Traps is now a passive skill.